In multiple access communication systems, carrier frequencies or time slots are generally utilized as communication resources between plural communicating units, and plural communicating channels are simultaneously required for communicating therebetween.
Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) are well known techniques in the multiple access systems.
The typical example of the FDMA system is so called Single Channel Per Carrier (SCPC)-FDMA system, in which a carrier frequency is selected from pooled vacant frequencies to be used as a communicating channel. On the other hand, the TDMA system is the system in which a time slot is selected from pooled vacant time slots to be utilized as a communicating channel.
In these systems, the connection and disconnection of communicating channels are done by using a common signalling channel, separately disposed from the communicating channels. Namely, in the SCPC-FDMA system, a specified carrier frequency, which is different from the communicating channel carrier frequencies, is allotted as the common signalling channel, and in the TDMA system, a specified time slot, which is different from the communicating time slots, is allocated to as the common signalling channel.
These conventional systems have the following problems. Namely, in the SCPC-FDMA system, when the communicating unit "A", which is communicating with another communicating one "B", desires to communicate with further another communicating unit "C", the communicating unit "A" must simultaneously transmit two carriers composed of a communicating channel carrier between the unit "A" and the unit "B", and a common signalling channel carrier for newly occurred channel request, or a new communicating channel carrier between the unit "A" and the unit "C".
When the plurality of carrier frequencies are simultaneously transmitted, a transmitting amplifier simultaneously amplifies a plurality of signals of carrier frequency, which is a multiple frequency amplifying operation. Accordingly, it is necessary to use a linear operating region with the transmitting amplifier having a large operating margin so as to avoid the generation of a distortion of intermodulation product caused by nonlinearity in operation of the transmitting amplifier. In such a situation, the transmitting amplifier is used in a state in which the efficiency is ultimately low. Accordingly, it is necessary to use a transmitter having a large electric power to secure a predetermined transmitting power in accordance with link budget, thereby increasing the cost of the system.
In the TDMA system, traffic between a plurality of communicating units is treated by the common signalling channel and the communicating channel at one frequency so that the transmission rate is ultimately high and the occupied band is increased. Accordingly, it is necessary to use a transmitter having a large power output and an antenna having a large diameter to transmit signals in a wide band in a predetermined power density. Further, digital circuits disposed in control sections, etc., of the TDMA system becomes complicated and the cost of the system is greatly increased.
As mentioned above, in the SCPC-FDMA system and the TDMA system known as the conventional multiple access communication system, when the simultaneous communication with a plurality of partners can be performed, it is necessary to use an expensive transmitter having large output power, thereby increasing the cost of the system.
Further, in the SCPC-FDMA system, the transmission rate of one channel allocated to one carrier frequency is fixed so that a plurality of carrier frequencies must be used to communicate at a carrier speed faster than a predetermined carrier speed and simultaneously communicate with a plurality of communicating units in the bi-direction.
In the TDMA system, a plurality of time slots can be used from multiplexed time slots so that it is possible to simultaneously communicate with a plurality of communicating units in the bi-direction at various speeds. However, this system is an uneconomical system for a user's communicating unit communicated at a low speed. Namely, in the TDMA system, a number of communicating channel time slots are fixedly multiplexed at one carrier frequency so that the transmission rate is much faster than in the SCPC-FDMA system. When the transmission rate is fast, the receiving noiseband is widened and the receiving channel quality is reduced so that it is necessary to increase the transmitting power and use an antenna having a large diameter, thereby making the TDMA control complicated and increasing the cost of the user's communicating unit.
As mentioned above, in the SCPC-FDMA system, it is economical for a user treating only one communicating channel at a predetermined speed, but it is not suitable for communications at various speeds corresponding to simultaneous bi-directional communications with a plurality of communicating units or multi-media which have been required by users. In contrast to this system, in the TDMA system, it is easy to perform simultaneous bi-directional communications with a plurality of communicating units or communicate at various speeds, but the user's communicating unit is expensive and, in particular, the communicating unit is greatly uneconomical for a user who do not need the simultaneous bi-directional communication with a plurality of communicating units. Further, a number of communicating units are disposed in the TDMA system so that the multiplex is increased as the traffic amount is increased, thereby increasing the cost of the user's communicating unit irrespective of the user's requirement.